Following traumatic brain injury and stroke, the normal response of the surrounding brain is to mount a cellular response that includes formation of reactive astrocytes that are believed to be important to “contain” and “clean-up” the injury site. Swelling of neural cells is part of the cytotoxic or cell swelling response that characterizes brain damage in cerebral ischemia and traumatic brain injury, and is a major cause of morbidity and mortality. See, Staub et al., 1993; Kimelberg et al., 1995. A number of mediators have been identified that initiate swelling of neural cells, including elevation of extracellular K+, acidosis, release of neurotransmitters and free fatty acids. See, Kempski et al., 1991; Rutledge and Kimelberg, 1996; Mongin et al., 1999. Cytotoxic edema is a well-recognized phenomenon clinically that causes brain swelling, which worsens outcome and increases morbidity and mortality in brain injury and stroke.
Mechanisms underlying apoptotic death of reactive astrocytes have been studied. See, Tanaka et al., 2000; Yu et al., 2001. The mechanisms responsible for necrotic cell death have not been characterized. Apoptotic cell death is preceded by cell shrinkage and net loss of K+. See, Yu et al., 1997; Yu et al., 1999. By contrast, in necrotic cell death, the plasma membrane is ruptured, causing cytosolic contents to be released and thereby triggering tissue inflammation. See, Leist and Nicotera, 1997. Necrotic cell death may be more deleterious to nearby viable tissues, given the secondary inflammatory damage that is initiated.
Necrotic cell death is initiated by osmotic swelling following influx of Na+, the major extracellular osmolyte. In most cell types, accumulation of Na+ intracellularly is regarded as a passive process that does not require activation of specific effectors but that is due instead to defective outward Na′ pumping under conditions of low [ATP]i. See, Leist and Nicotera, 1997; Trump et al., 1997. Cell blebbing or swelling, an indication of intracellular Na′ overload, is generally regarded as an early sign of necrotic cell death. See, Leist and Nicotera, 1997; Majno and Joris, 1995.
Inhibition of ATP synthesis or ATP depletion also causes neural cell swelling, blebbing and, if sufficiently severe, plasma membrane disruption and cell death. See, Jurkowitz-Alexander et al., 1993. The mechanisms of neural cell swelling associated with ATP-depletion remained incompletely characterized. See, Lomneth and Gruenstein, 1989; Juurlink et al., 1992; Rose et al., 1998.
One potential mechanism would be changes in Na+ and K+ concentration due to inhibition of the Na+/K+-ATPase pump. However, an equivalent degree of osmotic swelling induced by ouabain-mediated inhibition of the Na+/K+-ATPase pump in neural cells does not produce large depolarization, blebbing or cell death. See, Jurkowitz-Alexander et al., 1992; Brismar and Collins, 1993. Failure of the Na+/K+-ATPase pump, therefore, is not the mechanism critical to swelling of neural cells. None of these studies have identified the cellular mechanism instrumental in the cell swelling that is associated with brain damage in cerebral ischemia and traumatic brain injury.
One subtype of ATP sensitive cation channel is the non-selective cation channel, which are channels that are sensitive to Ca2+ and ATP. More specifically, non-selective cation channels are activated by intracellular Ca2+ ([Ca2+]I and inhibited by intracellular ATP ([ATP]i). Although Ca2+ and ATP sensitive cation channels had been identified in a number of non-neural cell types, they have not been identified in astrocytes or any other neural cells. See, Sturgess et al., 1987; Gray and Argent, 1990; Rae et al., 1990; Champigny et al., 1991; Popp and Gogelein, 1992; Ono et al., 1994, each of which is hereby incorporated by reference in its entirety. These non-astrocyte channels comprise a heterogeneous group with incompletely defined characteristics. They exhibit single-channel conductances in the range of 25-35 pS, discriminate poorly between Na+ and K+, are impermeable to anions, for the most part impermeable divalent cations, and they are blocked by similar concentrations of the adenine nucleotides ATP, ADP and AMP on the cytoplasmic side. The function of these non-selective ATP sensitive cation channels in these non-neural cell types remains enigmatic, in part because unphysiological concentrations of Ca2+ are generally required for channel activation.
Another subtype of ATP sensitive cation channel is the ATP-sensitive potassium channel (KAP channels) in pancreatic β cells. One class of insulin secretagogues, the antidiabetic sulfonylureas, are used to inhibit these KATP channels and stimulate insulin release in diabetes mellitus. See, Lebovitz, 1985. Antidiabetic sulfonylureas mediate their effect on KATP channels via a high affinity sulfonylurea receptor (SUR). See, Panten et. al., 1989; Aguilar-Bryan et. al., 1995. Several isoforms of the SUR, termed SUR1, SUWA, S W B, and SUR2C, have been identified and cloned. See, Aguilar-Bryan et. al., 1995; Inagaki et. al., 1996; Isomoto et. al., 1996; Lawson, 2000. These receptors belong to the ATP-binding cassette (ABC) transporter family, of which the cystic fibrosis transmembrane conductance regulator (CFTR), another ion channel modulator, is also a member. See, Higgins, 1992; Aguilar-Bryan et. al., 1995. Notably, the CFTR has major therapeutic importance, since its genetic absence causes cystic fibrosis, a fatal disease.
The sulfonylurea receptor imparts sensitivity to antidiabetic sulfonylureas such as glibenclamide and tolbutamide. Also, SUR is responsible for activation of the potassium channel by a chemically diverse group of agents termed K+ channel openers (SUR-activators), such as diazoxide, pinacidil, and cromakalin. See, Aguilar-Bryan et. al., 1995; Inagaki et. al., 1996; Isomoto et. al., 1996; Nichols et. al., 1996; Shyng et. al., 1997b. In various tissues, molecularly distinct SURs are coupled to distinct channel moieties to form different KATP channels with distinguishable physiological and pharmacological characteristics. The KATP channel in pancreatic β cells is formed from SUR1 linked with a K+ channel, whereas the cardiac and smooth muscle KATP channels are formed from SUR2A and SUR2B, respectively, linked to K+ channels. See, Fujita ad Kurachi, 2000.
Thus, a need exists for a physiological target instrumental in the cell swelling that is associated with brain damage in cerebral ischemia and traumatic brain injury and in the consequent morbidity and mortality. There is also a need for specific treatments for the cytotoxic edema that causes brain swelling, which worsens outcome and increases morbidity and mortality in brain injury and stroke. Also there exists a need for therapeutic compounds capable of modulating the activity of this target in order to prevent brain damage. The present invention is directed to a newly characterized non-selective calcium and ATP sensitive monovalent cation channel, termed the NCCa-ATP channel, which is present in neural cells and linked to an SUR. The present invention further provides a method to screen for or identify antagonists to NCCa-ATP channel activity. Further, the present invention provides a method for the therapeutic use of antagonists, such as sulfonylureas and other SUR1 blockers, to inhibit this channel's activity and thereby prevent neural cell swelling and cell death and the concomitant nervous system damage that includes brain swelling and brain damage.